1. Field of the Invention
The present invention relates to a double crucible for a glass drawing method with a heated outer crucible and an inner crucible surrounding the outer crucible. The inner crucible is formed so that it is heatable separately from the outer crucible. Both inner and outer crucibles have outlet nozzles for the glass to be drawn. The present invention also relates to a method of making glass fiber or a preform of the glass fiber with the double crucible.
2. Description of the Related Art
Core-clad glass fibers, i.e. glass fibers, which have an inner core comprising a first glass and at least one outer coating or cladding comprising a second glass, are typically drawn from a coaxial double crucible. The cladding or coating glass is located in the outer crucible and the core glass is located in the inner crucible.
In the known manufacturing methods typically double crucibles with heatable outer crucibles are used.
However if a glass fiber is made using a glass pair, whose coating glass located in the outer crucible has a lower viscosity than the core glass in the inner crucible, then control of the drawing process has proven to be difficult or impossible. In order to obtain a predetermined viscosity of the core glass, the outer crucible must be heated so that the viscosity of the coating glass would be too small.
Furthermore poor heat conduction and/or poor transparency during radiant heating of the glass in the outer crucible can make a sufficiently thorough heating of the contents of the inner crucible difficult.
A known double crucible with a heated outer crucible and an inner crucible surrounded by the outer crucible, in which the inner crucible is heated separately from the outer crucible, is described in DE 26 29 658 A. The core glass in the inner crucible may be brought to a higher temperature independently of heating the coating glass in the outer crucible. Thus a lower viscosity of the core glass may be adjusted to the viscosity of the coating glass.